Radio communication systems may have a cellular architecture, with each cell corresponding roughly to a geographical area. Each cell includes a base station (BS), which is a local central cite through which a number of radio transmitter/receiver units (user terminals (UTs)) gain access to the communications system. The UTs could be, for example, telephones, PDAs, or small modem boards. A UT establishes a communication link with other UTs by requesting access to the system through the BS. Each UT communicates over a communication channel distinguished from other UTs.
Various techniques exist to increase the number of available channels for a given number of available frequencies. Time division multiple access (TDMA), for example, divides a single frequency into multiple time slots. Each of the time slots can then be allocated to a separate communication channel. Other known techniques include code division multiple access (CDMA) and frequency division multiple access (FDMA), which, like TDMA, are considered conventional multiple access (multi-access) channel schemes.
Radio communications systems may employ a spatial division multiple access (SDMA) scheme, in conjunction with one or more conventional multiple access schemes, to increase the number of UTs that a BS can serve for a given number of available frequencies. An SDMA scheme may be implemented using a BS that has an array of receiver antenna elements. The antenna elements are spaced, one from another, typically about a half of a meter apart. The array of antenna elements introduces a spatial dimension that can be used to differentiate two or more UTs concurrently accessing a given conventional channel. That is, the basis of an SDMA system is that the BS creates a spatially distinct SDMA channel for each of multiple users even though they share the same carrier frequency (FDMA), timeslot (TDMA), or spreading code (CDMA).
This is done by weighting the uplink signal (communications from a UT to a BS) from each antenna element in amplitude and phase by a spatial demultiplexing weight (receive spatial weight), all the receive spatial weights determine a complex valued receive spatial weight vector which is dependent on the spatial signature of the UT. The spatial signature characterizes how the BS array receives signals from a particular UT. On the downlink (communications from the BS to a UT), transmission is achieved by weighting the signal to be transmitted by each array element in amplitude and phase by a set of respective spatial multiplexing weights (transmit spatial weights), all the transmit spatial weights for a particular UT determine a complex-valued transmit spatial weight vector which also is dependent on the spatial signature of the UT. When transmitting to several UTs on the same conventional channel, the sum of weighted signals is transmitted at the antenna arrays.
The weighting of the signals either on the uplink from each antenna element in an array of antennas, or on the downlink to each antenna element is referred to as spatial processing. The term SDMA channel is used to refer to each of multiple, spatially distinct channels of a conventional channel.
Personal Handyphone System (PHS)
A PHS is a TDMA-based system operating in the 1.88 GHz-1.93 GHz band and providing dynamic channel allocation. A UT of such a system uses a Traffic Channel (TCH) to communicate with the BS. If the TCH in use (i.e., the original channel) deteriorates, the UT will attempt to switch to another channel (destination channel). The switch may be to another TDMA channel provided by the same BS as the original channel. Such a switch is referred to as a TCH switch (channel switch). Alternatively, the switch may be to another BS, and if so, is referred to as a handover.
An attempted switch is not always successful. For various reasons it may not be possible to establish the communication link on the destination channel. For such situations it may be desirable to reestablish the communication link on the original channel rather than terminate the communication link. This process of reestablishing the communication link on the original channel is referred to as switchback.
Switchback
FIG. 1A illustrates a process in which a UT effects a switchback to an original channel on a PHS after an attempted channel switch failure in accordance with the prior art. As shown in FIG. 1A, the BS uses the original channel to broadcast a synchronization burst as the UT attempts a switch to a destination channel. This downlink synchronization burst provides a mechanism for the UT to reestablish a communication link on the original channel if the attempted switch fails. The synchronization burst is broadcast repeatedly throughout the attempted switch and switchback process. Upon failure of the attempted switch to the destination channel, the UT attempts to locate the downlink synchronization burst from the BS. The downlink synchronization burst indicates to the UT that a communication link can be reestablished on the original channel. Upon receiving the downlink synchronization burst, and determining that it can receive downlink synchronization bursts correctly, the UT broadcasts its own synchronization burst to the BS. This “handshaking” reestablishes a communication link for the UT on the original channel.
For a conventional PHS the switchback is not problematic because the TDMA timeslot of the original channel is vacant at the time of switchback and therefore both the downlink synchronization burst and the uplink synchronization burst can be received without interference. However, this is not the case for an SDMA PHS in which each timeslot accommodates multiple spatially distinct channels.
FIG. 1B illustrates a drawback of an SDMA PHS in the context of UT switchback in accordance with the prior art. As shown in FIG. 1B, UT A is one of multiple UTs assigned to a given TDMA channel (original channel) on one of multiple SDMA channels of the original channel. When UT A attempts a switch to the destination channel the BS begins broadcasting the synchronization burst to UT A on the original channel in preparation of a possible switchback. During this time, UT B is communicating with the BS over the original channel (albeit a spatially distinct channel of the original TDMA channel). The downlink TCH burst from the BS to UT B may cause severe interference with the downlink synchronization burst from the BS to UT A. That is, because the spatial signature of UT A is not known, the BS cannot focus the signal energy of the downlink synchronization burst to the extent necessary to overcome interference from the remaining TCH signals. In such cases, the handshaking between the BS and the UT attempting switchback (e.g., UT A) fails and the communication link for UT A may be terminated.